This is based on 1) Apple being secretive about the chipset and not bragging like other companies would 2) "multiple sources who are certain for different reasons that this is indeed the case."

First things first. Apple being secretive proves nothing. Apple is secretive about everything, and they've said before they don't discuss chipsets because they don't think it matters to consumer electronics users (and, hey, Apple is secretive). They didn't talk at all about the iPhone 3GS chipset -- all they said is it was twice as fast. We only know that it uses an ARM Cortex A8 because people bought it and tore it apart to find out.

As to the multiple sources, if one of the "different reasons" is direct knowledge of the chipset, then they could be exactly right. If not, then... eh, maybe, but we're back to waiting for the iPad to ship and people to tear it down.

In the interest of being complete, however, Ars speculates that if the iPad is indeed running the Cortex A8, this might be what's making it so fast:

it turns out that the the A4 is a 1GHz custom SoC with a single Cortex A8 core and a PowerVR SGX GPU. The fact that A4 uses a single A8 core hasn't been made public, but I've heard from multiple sources who are certain for different reasons that this is indeed the case. (I wish I could be more specific, but I can't.)

In all, the A4 is quite comparable to the other Cortex A8-based SoCs that are coming onto the market, except that the A4 has even less hardware. The iPad doesn't have much in the way of I/O, so the A4 itself can do away with the I/O that it doesn't need. In contrast, the typical Cortex A8-based SoC has more I/O hardware than a mobile phone can use, because you never know what customers will need which interface types.

Ars, like Venture Beat, also thinks Apple's PA Semi team may not be involved in the iPad because they're working on a variant for the 4th generation iPhone. Either way, they believe software will ultimately be more important than hardware -- which is something Apple's been saying for a while now.

So, do we care if the iPad has an ARM Cortex A8 rather than a Cortex A9? Do we want Apple pushing the hardware, or are we happy with them prioritizing software?